Light-emitting devices including an excitation light source that emits excitation light and a light-emitting unit that emits fluorescence in response to the excitation light have been developed.
An example of a light-emitting device is a light source device described in Japanese Unexamined Patent Application Publication No. 2012-89316 (published May 10, 2012). The light source device includes a solid light source that emits excitation light and a phosphor layer that emits fluorescence when excited by the excitation light emitted from the solid light source. The fluorescence emitted from the phosphor layer is used as illumination light. In this light source device, the shape and cross-sectional area of the excitation light beam on the incident surface of the phosphor layer are substantially the same as the shape and area of the entire incident surface of the phosphor layer. Since a sufficient amount of excitation light is incident on the incident surface of the phosphor layer over substantially the entire area thereof, the illumination light, in which the excitation light and the fluorescence are mixed at a certain ratio, is emitted from substantially the entire area of the incident surface of the phosphor layer.
However, in the light source device according to Japanese Unexamined Patent Application Publication No. 2012-89316, the intensity of the excitation light on the phosphor layer is not uniform. The intensity of the excitation light has, for example, a Gaussian distribution on the phosphor layer. This leads to the following problems.
Firstly, in the case where the intensity of the excitation light on the phosphor layer is not uniform, the intensity of the fluorescence emitted from the phosphor layer excited by the excitation light is also not uniform. Therefore, fluorescence having a nonuniform brightness distribution is emitted from the phosphor layer. Secondly, the phosphor layer receives a large amount of energy from the excitation light in a region where the intensity of the excitation light incident on the phosphor layer is the highest, and the temperature becomes high in that region as a result. Accordingly, temperature quenching and/or deterioration of the phosphor occur in that region, and the intensity of the fluorescence emitted from the phosphor decreases. As a result, the brightness distribution of the fluorescence emitted from the phosphor layer becomes nonuniform. Lastly, when the position of incidence of the excitation light on the phosphor layer is displaced or shifted, the intensity distribution of the fluorescence on the phosphor layer changes. In such a case, the intensity distribution and the shape of the fluorescence emitted to the outside from the light source device also change. This also makes the brightness distribution of the fluorescence nonuniform.